Surge limiters are regularly employed in modern electronic apparatuses and systems, such as in computer servers, telecommunications, and high reliability redundant systems. A surge limiter is generally designed to protect electrical equipment from power surges, or voltage or current spikes. In some applications, an alternating current (AC) voltage platform that supplies voltage to equipment in unconditioned form and may have power surges and dropouts based on power demand, load switching, and mission profiles. therefore, a properly designed surge limiter avoids deleterious effects to equipment voltage during, for example, the insertion or removal of circuit card assemblies into or from powered equipment. Generally, in these systems, the voltage source is a low impedance direct current (DC) distribution system, and it is typically well regulated and conditioned.
For example, an electrical system having high-power equipment (e.g., motors, compressors, an aircraft launcher, etc.) can require a lot of energy to switch the equipment on and off. The on/off switching can create a sudden, brief demand for power, which upsets the steady voltage flow in the electrical system, thereby creating power surges that are severe enough to damage downstream electronic components. In some situations, damage to the components can occur immediately, provided that a high enough voltage is provided to the component, even over a brief span of time (e.g., a voltage spike). In other situations, a relatively lower voltage can cause similar damage to downstream components if sustained for a longer span of time. Either situation can be detrimental to the electrical system.
Generally, a limiter uses a transistor to protect downstream equipment from upstream power surges. The pass transistor operates in one of two modes. During a power surge, the transistor operates in a shut-off mode (“off” mode), in which the current received from the upstream components is stopped and the excess voltage is blocked to the downstream components. At other times, the transistor operates in a saturation mode (“on” mode), in which the current received from the upstream components is passed through the transistor to the downstream electronic components.
Because the transistor blocks the energy received when operating in the “off” mode, the downstream components stop receiving power until the transistor switches back to the “on” mode. In some situations, such as when experiencing a brief voltage spike, the downstream components may be capable of continuing to operate for the duration of the spike. However, in other situations, such as where the surge is sustained for a relatively longer period of time, the downstream components may not be capable of continuing operation until the transistor switches back to the “on” mode. This is unacceptable in certain applications, in which it is desired that the components continue to operate even during surge conditions.
For example, in a power conversion unit (PCU) that converts an unregulated alternating current (AC) platform power into a filtered direct current (DC) power, the incoming AC voltage is unregulated and may have transient events, such as power surges and/or dropouts (e.g., based on power demands, load switching, mission profiles, etc.). These transients can generate high voltage DC during power surge conditions, which in turn can damage downstream components. In order to maintain operation of the downstream components during such conditions, at least some of the current from the PCU must be passed through to the downstream components, even during surge conditions.
Therefore, there is a need for a surge limiter that permits one or more downstream components to continue operation even during power surge conditions.